1. Field of the Invention
The present invention relates to a method and apparatus for producing hydrogen peroxide by injecting minute bubbles of hydrogen and oxygen into a liquid stream of an aqueous solution of water and an inorganic acid and a Group VIII metal catalyst in which the liquid stream flows at high velocity.
2. Description of the Related Art
It is known that a mixture of gaseous oxygen and gaseous hydrogen forms an explosive material. Virtually all currently produced hydrogen peroxide is produced by indirectly combining hydrogen and oxygen. Thus, the primary conventional industrial method for production of hydrogen peroxide uses a chemical agent first for the reduction or hydrogenation. Typically, alkylanthraquinone, such as ethyl or tertiary butyl anthraquinone, is used as the chemical agent. This working solution contains organic solvents such as di-isobutylcarbinol and methyl naphthalene. Oxidation of the intermediate product of the reduction reaction produces hydrogen peroxide and the original alkylanthraquinone. The anthraquinone is recycled back through the process. This method has the shortcoming that there is a considerable loss of the anthraquinone and the organic solvents from oxidation and thermal degradation of these organic materials. The presence of these organics with oxygen and hydrogen peroxide presents safety hazards from their potential reactions. This destruction of organics involves high operating losses for the currently practiced commercial process.
Various methods have been attempted to directly combine oxygen and hydrogen to overcome the above-described problems. The direct combination of hydrogen and oxygen to form hydrogen peroxide has the advantage of not requiring the use of any organic or combustible materials. The direct process, however, does require careful control of the gaseous mixture of oxygen and hydrogen so that they are always outside the explosive range.
There have been numerous patents issued for the direct combination of hydrogen and oxygen to produce hydrogen peroxide. U.S. Pat. Nos. 4,347,232 and 4,336,240 employ a two phase system through the use of organic compounds to form a second phase. However, the organics can react with oxygen or hydrogen peroxide to render these processes uneconomical.
Other patents employ a fixed bed catalyst within a reaction vessel which has low conversion per pass or low reactor productivity (see for example, U.S. Pat. Nos. 4,336,239 and 5,082,647).
U.S. Pat. No. 5,169,618 (""618 patent) to Marischino describes establishing a pulse-flow regime in a catalyst bed. The ""618 patent has the limitation of low conversion of hydrogen peroxide per pass and high equipment costs.
U.S. Pat. No. 4,996,039 describes first absorbing hydrogen into the aqueous reaction mixture with a catalyst; dropping the pressure to remove all the hydrogen in the gas phase; and then introducing the oxygen in order to produce hydrogen peroxide. This technique minimizes the presence of hydrogen in the gaseous phase. This process is expensive to repressure the reactor with oxygen and doesn""t lend itself to continuous processing.
Continuous modes of operation for the direct combination process have also been proposed, as disclosed in U.S. Pat. Nos. 4,009,252; 4,279,883; 4,681,751; and 4,772,458. These patents employ a catalyst as a slurry in an agitated reactor. These patents have the drawback of having either low conversion per pass or low volumetric efficiency either of which is uneconomical.
U.S. Pat. No. 4,661,337 (""337 patent) describes a process for direct combination for producing hydrogen peroxide of increased concentration. The volume of the reaction mixture occupies a small portion of the available volume of the reactor. It is taught in the ""337 patent that the layer of reaction mixture has a thickness of no more that 2 millimeters. This patent has the disadvantage that the majority of the reactor is in the gas phase in which no hydrogen peroxide is formed.
In all the above described patents, there is a separate continuous gaseous phase in which it is necessary to inject an inert gas such as nitrogen, argon or helium in order to remain outside the explosive range of hydrogen and oxygen.
Other attempts for the direct formulation of hydrogen peroxide use liquid filled reactors without a continuous gas phase. U.S. Pat. No. 5,104,635 describes a liquid filled reactor with two internal membranes which each are permeable only for hydrogen and oxygen, respectively. This reaction system requires considerable capital for the use of the membranes.
U.S. Pat. No. 4,279,883 describes a process for preparing hydrogen peroxide in an aqueous medium. The aqueous medium contains dissolved hydrogen and a platinum-group catalyst having absorbed thereto hydrogen. Inert nitrogen and argon are blown into the aqueous medium so that no dissolved oxygen is present in the aqueous medium during the hydrogen absorbing treatment. Oxygen gas is injected into the medium after the absorption of the hydrogen on the catalyst and the gaseous zone and liquid zones are stirred. This patent has the shortcoming of requiring an injection of an inert gas into the reactor during the hydrogen absorption phase to prevent an explosion between the hydrogen and oxygen gases.
U.S. Pat. No. 5,194,242 (""242 patent) describes a process for preparing hydrogen peroxide in which an acidic aqueous solution fills an elongated reaction zone in a tubular reactor. A catalyst is provided to the reaction zone. Oxygen together with recycled gas and then hydrogen are dispersed into the solution in proportions that are above the lower flammability limit for hydrogen and oxygen and are maintained at a temperature and pressure until the reaction mixture has decreased to below the lower flammability limit for the hydrogen and oxygen mixture. The partial pressure of hydrogen and oxygen is super-atmospheric in the range of about 20 to about 400 psi. The aqueous solution flows through the reactor at liquid velocity at rate from about 4 to about 18 ft/sec.
In the ""242 patent, the ratio of the flow of the aqueous medium to the aggregate flow of the hydrogen and oxygen is such that a gas phase regime of large elongated bubbles may be produced, which, if reacted violently would not be surrounded by sufficient liquid volume to cool the gas mixture, resulting in elevated temperature and pressure which can result in an explosion of the gas mixture. Patentees provide no teaching of the importance of operating in a regime in which small discrete individual bubbles exist which can be quenched by the surrounding medium. It is desirable to provide a safe direct combination process for producing hydrogen peroxide which has low manufacturing costs.
It is a primary object of the invention to provide a process for the reaction of oxygen and hydrogen which is efficient and safe.
It is a further related object of the invention to provide a process which operates in the flammable range of oxygen/hydrogen mixtures so as to benefit from increased reaction rate, without sacrificing the safety aspects of the process.
It is a further and related object of the invention to increase the molar ratio of gas (hydrogen and oxygen) relative to aqueous reaction solution in order to improve the space-time yield of the reactor system but without jeopardizing the safety of the reactor operation.
It is still a further related object of the invention to substantially completely react hydrogen during the process in order to maximize the efficiency of utilization of that expensive reactant.
It is still a further object of the invention to carry out the process with minute bubbles of hydrogen and oxygen supplied to the reaction zone at a rate and in such a way as to obviate the risk of explosion.
The invention comprises a method and apparatus for producing hydrogen peroxide in which hydrogen and oxygen are separately injected into a liquid filled reactor to form a plurality of discrete individual bubbles in a continuous rapidly flowing liquid stream. Each bubble is surrounded by a continuous liquid phase such that if the hydrogen and oxygen gas reacted, there is sufficient liquid available to quench/cool down the reaction in order to prevent an explosion propagating throughout the reactor. It has been found that it is critical to maintain the ratio of the volume of flow of aqueous medium to the aggregate volume of flow of hydrogen and oxygen, at a high value so as to avoid uncontrolled reaction of hydrogen and oxygen bubbles to form water. By controlling the ratio of the volume of flow of aqueous medium to the volume of flow of hydrogen and oxygen, both independently, and, in the aggregate, there is sufficient liquid volume present to quench any runaway reaction that might take place. It is also important to maintain the flow velocity of aqueous medium at at least ten feet per second to obtain a dispersed bubbly flow regime.
It has been found that the reaction can be safely and efficiently operated at higher levels of space-time yield if the reaction pressure is above 1200 psi. Preferably, the pressure is above 1500 psi. Most advantageously, it is from 2000 to 5000 psi.
Preferably, a pipeline reactor is used having a plurality of passes within the reactor. The pipeline reactor can be formed of a plurality of tubes arranged vertically or horizontally and connected with curved tubes (elbows). The liquid stream can be formed of water, a dilute acid and a Group VIII metal catalyst. The Group VIII metal catalyst can be platinum or palladium or a mixture of the two on an inert support such as alumina, silica or carbon. The liquid stream fills the reactor. Recycle gas containing hydrogen and oxygen is first injected into the flowing liquid stream. Fine dispersed hydrogen gas bubbles can then be dissolved into the flowing liquid stream. After the hydrogen is dissolved, finely dispersed oxygen gas bubbles are injected into the liquid stream for reacting with hydrogen to form hydrogen peroxide. After this first reaction is complete, multiple injections of first hydrogen and then oxygen can be used to raise the concentration of hydrogen peroxide produced to a predetermined level. The number of injections of hydrogen and oxygen bubbles can be varied for producing the desired concentration of the hydrogen peroxide.
It has also been found that the safety of the reaction system can be ensured if the reaction zone is comprised of vertically oriented pipes. The vertically oriented pipes prevent accumulation of gas bubbles at the top of horizontally oriented pipes that are very close or touching each other. If a reaction should occur within these bubbles, there is very little liquid around each bubble to permit cooling the bubbles. This could lead to an uncontrolled temperature rise with possible explosive results. Gas bubbles could also rise in vertical pipes, but the selected liquid velocity is sufficiently high so that the bubbles move continuously forward with flowing liquid medium.
It has also been found that it is advantageous to continuously cool the entire reaction zone throughout the duration of the reaction. In still a further improvement, it has been found that the step of separating unreacted gases from the aqueous reaction medium can be carried out by introducing air into the unreacted gases, rather than nitrogen, and thereby achieving greater economies of operation.
The present invention has the advantage of avoiding a continuous gas phase between the hydrogen and oxygen and having full utilization of the entire volume of the reactor. The production of hydrogen peroxide occurs in the liquid phase between the dissolved hydrogen gas and oxygen in the presence of a catalyst. The method prevents an explosive reaction from expanding, thereby allowing the concentration of unreacted hydrogen and oxygen to be within the explosive range.
The invention will be more fully described by reference to the following drawing.